"Amanda is very sad at this news but is strong and ready to fight on," said Carlo dalla Vedova, a lawyer representing her, who spoke to Knox in Seattle immediately after the verdict. "She thought the nightmare was over, but was ready for this after the discussion in the Supreme Court went on longer than predicted. This means further harrassment, but she ready to fight," he said.

"This is very heavy psychologically for Amanda," he added. "She told me she wanted to return to Italy, she loves this country, but she probably will not come now."

Knox's sentence for falsely accusing a Perugia barman, Patrick Lumumba, of the murder, was also upheld by the Supreme Court.

Francesco Maresca, a lawyer representing the Kercher family, punched the air in a sign of victory when he heard the verdict. “I am really happy. I had faith in the Supreme Court which is known for its preparation and crystal clear judgements," he said.

Maresca said he had informed Stephanie Kercher, Meredith's sister, of the decision. "She was very happy and a bit surprised," he said. Stephanie had planned to travel to Rome for the ruling but stayed in the UK to be near her mother who is unwell.

After spending four years in jail, Knox and Sollecito were acquitted on appeal in 2011 but prosecutors challenged that acquittal in Italy’s Supreme Court, as permitted by Italian law, which grants two levels of appeal. Ivory Coast-born drifter Rudy Guede was sentenced to 16 years for his role in the murder in a separate trial.

The prosecution’s appeal, which was backed by Kercher’s family, was upheld after a five hour hearing on Monday during which prosecutors and lawyers representing Knox and Sollecito battled over key evidence. The six judge panel will release the reasoning behind their verdict at a later date.

Knox will not need to return to Italy for the new appeal trial since defendants in Italy are not required to be present at their trials. Should she be convicted, she could face an extradition request from Italy, which would require the approval of the US government, said Dalla Vedova.

Sollecito, who is currently studying robotics in Verona and celebrated his 29th birthday on Tuesday, was “unlikely” to be jailed again in the run-up to the new appeal hearing, said Bongiorno.

At the hearing on Monday, prosecutor Luigi Riello claimed the freeing of the American student was a “Violation of the law which must be annulled.”

“"I believe all the premises are there to make sure the final curtain does not drop on this shocking and dire crime," he told the judges.

Under Italian law, Supreme court judges do not hear evidence but check verdicts for any procedural or technical errors.

But what should have been a quick hearing rapidly turned into a hard fought battle, as prosecutors and lawyers ignored protocol to pour over the finer details of the seven year old case.

Bongiorno said it was possible the Supreme court judges had found fault with just one aspect of the appeal court ruling that had freed Knox and Sollecito. “As such, when the case returns to the appeal stage the court may be asked to consider just that aspect, rather than restaging the entire appeal. We will have to wait for the judges’ motivations to be released to find out,” she said.

Bongiorno said that after a new appeal hearing, the case would once again return to the Supreme court. “But it is unlikely the Supreme Court would overturn the new appeal court ruling, provoking an infinite game of ping pong,” she said.